1. Field of the Invention
The present invention concerns a magnetic resonance apparatus of the type having a radio-frequency shield at a reference potential, a detuning circuit and an electrical conductor connected to the detuning circuit.
2. Description of the Prior Art
Modern magnetic resonance (MR) apparatuses normally operate with a number of different antennas (also called coils in the following) to radiate radio-frequency pulses (RF pulses) for exciting nuclear magnetic resonance and/or for receiving the induced magnetic resonance signals. An MR apparatus typically has a larger, whole-body coil, also called a body coil (BC), normally integrated in a fixed manner in the apparatus, as well as a number of small local coils (LC), also called surface coils. In contrast to the whole-body coil, the local coils serve for obtaining a detailed image of body parts or organs of a patient that are located relatively near to the body surface. For this purpose, the local coils are applied directly to the location of the patient at which the region to be examined is located. Given the use of such a local coil, in many cases the exciting signal (energy) is emitted by the whole-body coil (as a transmitter coil) fixedly integrated into the MR apparatus and the induced MR signals are received with the local coil (as a receiver coil). So that the coils do not interact with one another, it is necessary to detune the receiver coil in the transmission phase and the transmitter coil in the reception phase. Upon detuning, the eigenresonance (self-resonance) frequency of the respective antenna is shifted such that it no longer lies in the range of the operating MR frequency. An antenna shifted in this manner behaves neutrally in the ideal case, i.e. it is transparent to the RF pulses emitted by the other coils, or the induced MR signals. Insofar as there is a constant switching back and forth between two different antennas, this temporary shifting in the transmission phase and the reception phase is called a “dynamic detuning”. A coil also can be permanently detuned, insofar as it should only be operated with another coil. Such a “static detuning” is in particular necessary when a transmission-capable local coil is used that assumes both the transmission and the reception functions. Since the larger whole-body coil integrated into the MR apparatus cannot be physically removed during the imaging data acquisition, it is electrically deactivated by the detuning.
A number of magnetic resonance antennas that exhibit a structure known as a birdcage structure are known for use as a whole-body coil. Such an antenna has a number of longitudinal antenna rods running in parallel and arranged on a cylindrical surface, the longitudinal antenna rods being respectively connected to one another at the ends in a radio-frequency manner via an antenna ferrule. The longitudinal antenna rods and antenna ferrules in principle can be fashioned in an arbitrary form. In many cases, they are conductor strips that are applied on a flexible conductor strip or foil that is cylindrically wound around the measurement space in which the examination subject is located during the examination. In a whole-body coil, the birdcage structure runs around the patient acceptance chamber in which the patient is positioned during the exposure. In local coils in the form of a birdcage structure, the measurement space serves for the acceptance of the head or other extremities of a patient in order to precisely examine that region.
In principle there are various possibilities to detune such MR antennas with a birdcage structure.
Insofar as the field strength of the base magnetic field (also called the B0 field in the following) of the magnetic resonance system is below two Tesla, a detuning is very easily possible via the radio-frequency feed line. A shortcircuit is thereby benerated at the coil-distal end of the feed line to detune the antenna by means of a suitable switching element, for example a PIN diode or a relay. This short is transferred via the feed line to the feed point, i.e. the connection point at which the feed line is connected to the antenna. The detuning thereby achieved is sufficient to suppress coupling with the other active antenna. The advantage of such a coildistal detuning is that the supply line for the direct current that is necessary for the switching elements can be easily realized since, due to the large separation, no interaction of the switching direct current with the high static and radio-frequency fields occurs in the immediate surroundings of the antenna.
At higher B0 field strengths, however, the coil-distal detuning is not reliable. In such cases it is necessary to integrate the detuning elements directly into the structure of the antenna. In the detuning of an antenna with a birdcage structure, this can occur either by the antenna ferrules or the longitudinal antenna rods, or both, being detuned. Given such a detuning, a resonant inductor generally is interrupted or a resonant capacitor is bridged, i,e. shorted, by means of a suitable radio-frequency switching element. Switching diodes, for example PIN diodes, are currently used as RF switching elements since these are able to withstand both high currents and high voltages at high frequency and additionally can be switched sufficient rapidly.
Ferrule detuning in a birdcage structure is advantageous because easy access to the detuning elements, i.e. the RF switching elements, is possible from the outside, so the necessary direct current supply lines can be suitably placed without problems. In terms of radiofrequency, such a ferrule detuning in a birdcage structure, however, is not the optimal solution. Antennas with birdcage structures therefore are known in which the radio-frequency switching elements are arranged within the longitudinal rods, i.e. longitudinal inductors, with which radio-frequency switching elements in the appertaining longitudinal rods can be interrupted and thus the entire rod structure can be detuned. Each RF switching element is individually fed with the necessary direct current signals from outside, i.e. from outside of the birdcage structure. A significant disadvantage is that these supply lines must be placed through the resonant structure Without the antenna being disturbed with regard to its radio-frequency function. Each individual direct current supply line must therefore be individually choked and decoupled, for which special, very complicated condutors are used. Aside from the fact that the spatial relationships inside the iresonant structure are crowded and only components that are difficult to access can be accepted for the coupling and decoupling, this design is very costly due to its high production expenditure.
A circuit arrangement to control a radio-frequency switching diode is known from German PS 44 22 069, in which one diode is arranged electrically antiparallel (with opposite polarity) to the radio-frequency switching diode. A control signal to activate the radio-frequency switching diode can be generated from a radio-frequency current to be switched by the radio-frequency switching diode. A capacitive element is electrically connected in series with the diode. The capacitive element is bridged with a switch that can be electrically controlled by a control input. A control unit is connected with the control input. The control unit outputs an activation signal to the controllable switch. The diode can generate the control signal to activate the radio-frequency switching diode.
From German OS 44 14 371, a magnetic resonance apparatus is known in which a radio-frequency shield is arranged between a radio-frequency antenna and a gradient coil system of the magnetic resonance apparatus. The radio-frequency shield is fashioned such that it is permeable to the electromagnetic fields generated by the gradient coil system and impermeable to the fields in the radio-frequency range generated by the radio-frequency antenna. The radiofrequency shield has a first electrically-conductive layer arrangement and a second electrically-conductive layer arrangement opposite the first, the layer arrangements being separated from one another by a dielectric. The layer arrangement have conductor paths arranged next to one another that are separated from one another by electrically-insulating slits. The slits in the first layer arrangement are offset relative to those in the second, and in at least one layer arrangement adjacent conductor paths are connected with one another by specially arranged bridges (for example formed by capacitors) conducting radio-frequency current.
A radio-frequency structure with a force generator is known from German OS 102 28 827. The force generator can adjust a predetermined shape of the radio-frequency structure. For this purpose, actuators that are fashioned as copper conductor runs are activated by an electrode structure and are simultaneously used as antenna conductors to transmit radio-frequency signals and to receive magnetic resonance signals.